Fig. 12

Working model of how the glucose dual-affinity transport system functions in glucose transport, signaling, and CCR. Under high levels of glucose, N. crassa predominantly uses the low-affinity, but high-capacity GLT-1 protein for nutrient assimilation. Adequate glucose stimulates CRE-1-mediated CCR to repress cellulase expression for non-preferred carbon (e.g., plant cell walls) utilization, whereas the cAMP-PKA pathway is induced for vegetative growth. When external glucose is depleted or limited, however, HGT-1/-2 are rapidly derepressed by the lifting of CCR and transport the limited external glucose. This process synergistically downregulates the cAMP-PKA pathway via one or more mechanisms to decrease cAMP levels and pkac-2 expression, thus leading to attenuated glycolytic activity. Constant exposure of cells to glucose-limited conditions leads to sexual sporulation for survival, thereby also resulting in downregulation of clr-1/2 and cellulase expression, and reactivation of CCR on cellulose. Whether cAMP-PKA can regulate CRE-1 and/or CLR-1/-2 directly or through downstream transcriptional factors to function in plant cell wall deconstruction requires further investigation